1. Field of the Invention
The present invention relates to a ball screw which has a structure in which a screw shaft is linked to a nut member by a large number of balls constrained to roll in a space formed therebetween, and converts rotational motion of a motor to linear motion to carry out power transmission in a sliding member of a machine tool or a robot for industrial use, and more particularly to an improvement in structure for causing the above-mentioned balls to be endlessly circulated in the inside of the nut member.
2. Description of the Related Art
Such a kind of ball screw composed of a screw shaft having on its outer periphery a helical ball running groove, and a nut member to which the screw shaft is linked by a large number of balls constrained to roll in a space formed therebetween. Rotation of any one of the screw shaft and the nut member causes relative linear motion of the nut member to the screw shaft, with the result that the balls are endlessly circulated in the inside of the nut member.
There have been proposed many kinds of ball screw having nut members, which were different from each other in structural component for endless circulation of the balls therein. As one of the proposed ball screws, it is known the ball screw disclosed in Japanese Utility Model Provisional Publication No. S63-123,856, which comprises a screw shaft having on its outer periphery a helical ball running groove; a nut member having a hollow cylindrical shape so as to cause the screw shaft to be inserted therein, the nut member having on an inner surface thereof a helical ball running counter-groove, which faces on the helical ball running groove of the screw shaft, and the nut member having a pair of ball-passing holes formed apart from each other on the ball running counter-groove, for connecting inner and outer surfaces of the nut member with each other; a large number of balls arranged between the ball running groove of the screw shaft and the ball running counter-groove of the nut member so as to be moved under a prescribed load imparted thereto; a ball returning means arranged on the outer surface of the nut member, and having a no-load passage for connecting the ball-passing holes with each other for circulation of the balls; and a pair of deflectors each formed into a helical-shape, and fitted into the ball running counter-groove of the nut member so that the deflector is disposed in a prescribed position, having a corresponding relationship of the inside surface of the deflector to the ball-passing holes of the nut member.
In the ball screw having the above-described structure, both ends of the ball running counter-groove of the nut member are connected to each other by the no-load passage formed in the ball returning means and by the pair of ball-passing holes of the nut member. As a result, the revolution of the nut member around the screw shaft causes the ball to go in and come out of these ball-passing holes so as to endlessly circulated in the passage formed between the ball running groove of the screw shaft and the ball running counter-groove of the nut member and in the no-load passage of the ball returning means. The inside surface of the deflector fitted into the above-mentioned ball running counter-groove of the nut member is placed directly below the ball-passing hole. When the ball running under a prescribed load imparted thereto between the ball running counter-groove of the nut member and the ball running groove of the screw shaft comes in the vicinity of the ball-passing hole, a collision of the ball with the inside surface of the deflector accordingly occurs, and then the ball is picked up from the ball running groove of the screw shaft so as to be transferred into the no-load passage of the ball returning means through the above-mentioned ball-passing hole.
In the conventional ball screw having the above-described structure, the deflector is merely fitted into the ball running-counter groove of the nut member under the function of resiliency of the deflector. The repetitious collision of the ball with the inside surface of the deflector along with the endless circulation of the balls cause the deflector to be gradually moved to change its setting position, thus leading to a difficulty in that a precise positional relationship of the inside surface of the deflector with the ball-passing hole of the nut member cannot be maintained. As a result, there may occur problems such that it becomes impossible to smoothly transfer the balls picked up from the ball running groove of the screw shaft into the ball passing hole of the nut member, the smooth rotation of the nut member relative to the screw shaft is hindered by an increased friction resistance due to deterioration of the ball circulation, and a noise is made during rotation of the nut member.
When there is used a deflector formed into a longer helical body to be fitted into the ball running counter-groove of the nut member, it is possible to firmly secure the deflector to the nut member so that the setting position of the deflector may not easily be changed by the collision of the balls with the inside surface of the deflector. In this case, it is however necessary to increase the length of the ball running counter-groove of the nut member accordingly, thus causing inconveniences of carrying out a time-consuming threading work for the formation of the ball running counter-groove on the nut member, and leading to impossibility of realization of making the nut member in a small size, when the ball running counter-groove has a longer lead.